Estrogens have widespread applications in health and disease. Postmenopausal women are treated with estrogen replacement therapy to relieve them of the menopausal symptoms. Between 1940-1975, an estimated 5-10 million Americans received diethylstilbestrol (DES), a synthetic estrogen, during pregnancy or were exposed to the drug in utero. Exposure to DES has been associated with an increased risk for breast cancer in DES mothers and a lifetime risk of cervicovaginal cancers in DES daughters. Exposure to DES has also been linked to a wide range of abnormalities in DES sons and daughters including immune system disorders such as increased incidence of autoimmunity, cancer and certain infections. Furthermore, a number of xenoestrogens which include environmental estrogens and phytoestrogens have been identified which bind estrogen receptors (ERs) and mediate activity similar to physiological estrogens. Thus, research on DES may serve as a template to study a growing list of endocrine disruptors. Studies from our lab demonstrated for the first time that prenatal and postnatal exposure to estradiol (E2) and DES can induce apoptosis in thymocytes, alter the expression of T cell receptor and CD molecules, and modulate positive and negative selection of T cells. In the current study, we will test the central hypothesis that prenatal exposure to DES alters the positive and/or negative selection of T cells in the thymus leading to generation of skewed T cell repertoire with enhanced reactivity towards self antigens and decreased response against non-self antigens. In aim# 1, we will investigate whether the regulation of Fas and Fas ligand genes involves estrogen responsive elements (ERE) and other transcription factors. Aim# 2 will test if prenatal exposure to DES alters positive and negative selection of T cells in the thymus and thereby skewing the immune responsiveness of mature T cells to self rather than non-self antigens. Aim# 3 will investigate the role of ERa in DES induced apoptosis and development of autoimmunity. Also, the role of T cell-stromal cell interactions in DES-induced thymocyte apoptosis will be studied. In Aim# 4, the mechanism by which prenatal DES exposure leads to increased susceptibility to autoimmune disease postnatally will be tested. Together, the studies proposed should provide novel information on the mechanism by which prenatal exposure to DES leads postnatally to increased susceptibility to autoimmunity and cancer and to development of potential approaches to prevent such immunotoxicity.